Regardless of where the black bishop moves, one of the two white rooks will succeed in obtaining a back rank mate, as the bishop must move and can no longer guard both a8 and e8 from the rooks. Also, there is no way for the bishop to check the king immediately, nor for the black king to free itself.

5.Re1? Be4!, similarly 5.Rf1? Bf3!
This composition is very interesting and not so easy to solve, e.g. 2.a7? is tempting but leads to nothing after 2...~ 3.a8=Q+ Bxa8 4.Qh1 Bxh1 because the c1 square is not cleared.
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Btw welcome _________________"Si tu vois un bon coup, ne le joue pas!!" - Philippe Bichu (after Em. Lasker)

There is no bug in the program because there is no program for endgame studies (only for #1/2/3).
The wrong reply 5...Bc6? to 5.Re1? had been entered by puzzle setter.
The solution is now edited._________________"Si tu vois un bon coup, ne le joue pas!!" - Philippe Bichu (after Em. Lasker)